Advances in combustion technology have employed high velocity gas injection, typically by an oxygen lance, into a furnace combustion zone to carry out combustion with reduced generation of nitrogen oxides (NO.sub.x). The injection of the gas at high velocity causes furnace gases to be aspirated or entrained into the injected high velocity gas stream and this reduces NO.sub.x generation.
Nozzles with relatively small diameters are usually employed to achieve the high velocity for the injected gas. One problem encountered with high velocity gas injection into a combustion zone is that the furnace gases, which may comprise particulate matter and condensable vapors, cause the lance nozzles, which typically have small openings, to foul or corrode easily as the furnace gases are aspirated or entrained into the high velocity gas exiting the lance nozzle. The furnace gases are quite hot, on the order of 2,000.degree. F. to 3,000.degree. F., and this exacerbates the nozzle fouling and corrosion problem.
One attempt to solve the nozzle problem involves using a large amount of water to cool the nozzle so as to prevent high temperature corrosion or nozzle melting. This approach has problems in that it is relatively complex to implement and operate. It also can escalate the nozzle corrosion and fouling problems when the furnace atmosphere contains condensable vapors. Another approach has been to use ceramic nozzles. However, ceramic nozzles, when used in highly condensable and high corrosive furnace atmospheres, become fouled and the ceramic is eroded away. For example, in an application in which a ceramic single hole nozzle was used in a borosilicate fiberglass furnace, the experience was that the nozzles had to be replaced once a week. An acceptable nozzle replacement rate should be no more than once every six months.
U.S. Pat. No. 5,295,816, made progress in solving the problem by using a low velocity protective gas surrounding the nozzle. The protective gas, which can be oxygen (used with an oxygen lance), preferably comprises 10 to 30% of the total gas flow, and has a low velocity preferably 10 to 50 ft/sec around the main gas jet so as to be entrained into the main gas jet. While this system provides some protection for a metallic nozzle, it does not completely solve the problem of nozzle corrosion because furnace gases are drawn into the cavity and entrained into the high velocity main gas. Another approach to reducing the corrosion is to reduce the velocity of the main gas from the lance. But this results in higher NO.sub.x emissions from the burner with minimal effect on lance maintenance. A further approach is to cast, or core-drill, a hole in the burner block and use the hole as a nozzle to introduce staged oxygen into the furnace. Implementation of this approach results in increased maintenance of the furnace since the oxygen from the hole cools the entrance to the furnace wall passage in which the lance is located. It also causes nozzle corrosion and formation of tubes of condensables. In addition, this approach creates inflexibility because it does not permit the lance oxygen velocity to be changed for the same flow rate.
The aforesaid problems become particularly acute in certain applications, such as an enamel frit glass furnace, where it is desired to reduce NO.sub.x emissions. Here the furnace flue gas atmosphere is highly corrosive and contains more condensables than found, for example, in a typical soda lime glass furnace. Prior experience with borosilicate glass furnaces, which have fewer corrosives and condensibles in the flue gas atmosphere than enamel frit furnaces, but more than a soda lime glass furnace, resulted in high wear and maintenance for an oxygen lance ceramic/stainless steel composite nozzle. This was so even when using protective gas techniques such as described in the aforesaid U.S. Pat. No. 5,295,816.